In obtaining thrust from a rocket motor, the constricted section of the exhaust nozzle or throat section is designed to provide a predetermined exhaust gas flow rate at a defined combustion chamber pressure. After the exhaust gases exit the throat section, the gases are expanded through an expansion exit cone to provide thrust based on the total rocket motor design. The throat area and the other sections of the exhaust nozzle must be able to withstand the high temperatures of the exhaust gases; otherwise, erosion by the hot exhaust gases, particularly in the throat area, will cause change in the combustion chamber pressure and burning rate and the total thrust delivered.
State-of-the-art materials which have been used in nozzle throat fabrication are; carbon-phenolic composites, carbon-carbon composites, bulk graphite, pyrolytic graphite, etc. Generic carbon-carbon composites perform well when exposed to motor exhaust gases which are at low flame combustion temperatures, but are not adequate in a high flame temperature environment. The reason is that above 2000.degree. C. temperature environment carbon reacts with the water and carbon dioxide which are present in the exhaust gases. The rate of these reactions increases as the temperature increases.
To reduce the erosion as a result of this chemical reaction, the state-of-the-art nozzle throats have been designed to have nozzle throats of very high temperature erosion-resistant metals, such as, rhenium, hafnium, tungsten, zirconium as coating materials. These have serious limitations, such as, nuclear sensitivity, brittleness, cracking, and flaking off.
A need exists for changing the surface of the carbon-carbon composite material of the nozzle throat area to enable it to withstand higher temperatures while retaining the desirable features of a carbon-carbon substrate backup material of the nozzle.
An object of this invention is to provide a method for converting the near surface of the carbon-carbon composite material at the surface of the interceptor nozzle throat to be more heat resistant.
A further object of this invention is to provide a method of manufacture for surface conversion of a carbon-carbon composite nozzle material to an erosion resistant surface.
Still a further object of this invention is to provide a method for the conversion of bulk graphite to a more heat resistant form to a more chemical-resistant form which is less subject to erosion in a rocket nozzle throat environment.